Once it gets to the lungs it can cause what is called a Cytokine storm. A severe autoimmune over reaction. This is when the immune system starts manufacturing way to many "T" cells to fight the virus and their activating proteins cytokines. This often associated with a surge of activated immune cells into the lungs. The resulting lung inflammation and fluid buildup can lead to respiratory distress and can be contaminated by a secondary bacterial pneumonia -- often enhancing the mortality in patients.
Ironically the more healthy you're immune system is, the more likely it is to kill you. Crazy thing is. This doesn't look like any other virus out there. It's not related to any virus known. It looks like someone took HIV and SARS and CRISPR'd them together.
At this point it's safe to say "who the hell knows what we're dealing with". Don't underestimate it.